1. Field of the Invention
The present invention relates to a device for acquiring information about displacement of an object. More particularly, the present invention relates to a device for detecting an origin serving as a reference position for counting the number of the periodic signals.
2. Description of the Related Art
Recently, a diffraction interference type encoder which measures a displacement of an object by measuring interference light generated from a plurality of orders of diffraction light diffracted by scales that relatively move has been used as a device for measuring a relative displacement of a mechanical stage or the like at a resolution of the order of sub-microns.
When a diffraction interference type encoder performs position measurement, it is generally performed that an origin (reference position) for a signal output from the encoder is detected, then a value of a counter is reset to zero, and an absolute position signal is output. Hitherto, detection of an origin is performed by the following methods. A first method is of obtaining an origin signal by illuminating a slit-like pattern with a converged light flux and generating a reflected light flux from the slit-like pattern like a pulse. A second method is of providing a plurality of slit arrays arranged at unequal intervals, irradiating light fluxes thereon to obtain patterns same as the slit arrays, collectively detecting reflection light, and generating a sharp pulse-like signal at a moment when patterns of a reflection portion of the slit arrays match contrast distribution patterns of the illumination light to the slit arrays, thereby obtaining an origin signal. A third method is of providing gratings with different periodical pitches side by side, generating a plurality of signals with different signal periods, and generating an origin signal at a position at which a phase difference among a plurality of signals is a predetermined value (e.g., zero). Japanese Patent Application Laid-Open Nos. 2004-144581 and 2004-170153 discuss the third method.
However, detection principles of the first and second methods utilize variation in amount of light due to geometrical optical blocking of a light flux. Thus, reproducibility of a pulse-like signal is several microns (μm) at most. Consequently, accuracy is insufficient for detecting an origin of a diffraction interference type encoder that outputs periodic signals having a period of the order of sub-microns.
The third method can provide higher detection accuracy than the first and second methods. However, the signal period of the diffraction interference type encoder is particularly short. Thus, a plurality of positions at which the phase difference among signals has a predetermined value is contiguously generated. Consequently, no origin can be identified. Accordingly, the third method is difficult to be put to practical use. Japanese Patent Application Laid-Open No. 2004-170153 discusses the following method as means for resolving this matter. According to the method, periodicity of a sine wave is changed among positions at which the phase difference between a plurality of signals is a predetermined value. Then, a position at which the periodicity has a value associated with an origin is detected. Thus, an origin is identified.
According to the method discussed in Japanese Patent Application Laid-Open No. 2004-170153, the interval between the positions at which the phase difference has the predetermined value is counted and stored by a counter. An origin signal is determined according to a state of the counted intervals. In this procedure, an analog signal output from a light receiving portion is to be converted into a digital signal by an analog-to-digital (AD) converter or the like, and then the digital signal can be processed. In addition, the digital signal is counted using a counter or the like, so that a calculation operation is complex. In such a calculation operation, calculation functions of a field programmable gate array (FPGA), a digital signal processor (DAP), a central processing unit (CPU) or the like, counters, and clock signals are to be obtained. In addition, it is difficult to detect an origin at high speed.